1. Field of the Invention
The present invention relates to an image forming apparatus of the type developing a latent image formed on an image carrier with a developer, which forms a magnet brush on a developer carrier.
2. Description of the Background Art
Generally, a copier, printer facsimile apparatus or similar electrophotographic or electrostatic image forming apparatus includes an image carrier implemented as a photoconductive drum or a photoconductive belt. A latent image is formed on the image carrier in accordance with image data. A developing device develops the latent image with toner to thereby produce a corresponding toner image. Today, magnet brush type development using a two-ingredient type developer, i.e., a toner and carrier mixture is predominant over development using a one-ingredient type developer, i.e., toner only. Magnet brush type development is desirable in the aspect of image transfer, reproduction of halftone, stable development against varying temperature and humidity, and so forth. The toner and carrier mixture rises on a developer carrier in the form of brush chains and feeds the toner to a latent image formed on the image carrier in a developing region. The developing region refers to a range over which the magnet brush on the developer carrier contacts the image carrier.
To further enhance image quality by development using the two-ingredient type developer, a halftone image portion must be freed from non-uniform toner deposition and granularity. A printer or a digital copier, for example, is required to uniformly form dots at the intervals of several ten micrometers for smoothly rendering halftone. In practice, toner fails to uniformly deposit over the area of a single a dot, as observed through, e.g., a microscope.
In light of the above, Japanese Patent Laid-Open Publication Nos. 7-114223 and 5-119592, for example, propose to use a bias for development that contains an oscillation component. Specifically, Laid-Open Publication No. 7-114223 discloses a method using a developer made up of toner having a mean grain size of 2 xcexcm to 6 xcexcm and carrier having a mean grain size of 45 xcexcm or below. This method selects an AC frequency of 6 kHz or above and develops a digital latent image having 350 pixels for an inch or above. Laid-Open Publication No. 5-119592 teaches a method that confines the grain size of toner in the range of 6 xcexcm and 11 xcexcm and uses an AC frequency between 3 kHz and 16 kHz and a peak-to-peak voltage between 1 kV and 2.2 kV. This method, according to the above document, frees the toner from the force of inertia that would prevent the toner from sharply following the variation of an electric field, thereby implementing faithful toner deposition on a latent image.
However, we found by experiments that the conventional methods described above did not improve image quality, but rather degraded it, depending on the bias. For example, when the bias had a peak-to-peak voltage of 1 kV or above as in Laid-Open Publication No. 5-119592, granularity was aggravated. On the other hand, even when the AC frequency was lower than 6 kV proposed by Laid-Open Publication No. 7-114223, granularity was improved in some conditions. The frequency of 6 kV or above was apt to blur the trailing edge of a solid image; the effect of the AC bias was practically lost when the frequency was limitlessly raised. The differences between the experimental results are presumably ascribable to the movement of toner and carrier that are susceptible to other various factors. It may safely be said that the grain size of toner, AC frequency and peak-to-peak voltage cannot easily reduce granularity alone.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 2000-305360 and 2001-5266.
It is an object of the present invention to provide an image forming apparatus capable of surely depositing toner on the individual dot at a nip to thereby form a uniform toner image and freeing a toner image on an image carrier from disturbance that brings about granularity.
In accordance with the present invention, in an image forming apparatus, the flux density of a main magnetic pole for development has an attenuation ratio of 40% or above in the normal direction is disclosed. Assume that aperiod of time of T1 is necessary for a photoconductive element to move by a single dot at a nip for development. Also, assume that, in a single period of an alternating electric field applied to a developing sleeve as abias, the duration of an electric field causing toner to move toward the photoconductive element is T2. Further, assume that the duration of an electric field causing the toner to move toward the sleeve is T3. Then, a relation of T1 greater than T2 greater than 0 or a relation of T1 greater than T3 greater than 0 holds.